This invention relates to gas separation membranes comprised of polyphosphazene polymers and processes utilizing such membranes for selectively separating acid gases from non-acid gas mixtures by permeation.
The separating, including upgrading or the concentration of at least one selective gas from a gaseous mixture, is an essentially important procedure in view of demands on the supplies of chemical feedstocks. Frequently these demands are met by separating one or more desired gases from gaseous mixtures and utilizing the gaseous product for processing. Applications have been made employing separation membranes for selectively separating one or more gases from gaseous mixtures. To achieve selective separation, the membrane exhibits less resistance to transport of one or more of the gases than of at least one other gas in the mixture. Thus, selective separation can provide preferential depletion or concentration of one or more desired gases in the mixture with respect to at least one other gas and, therefore, provide a product having a different proportion of the one or more desired gases to at least one other gas than the proportion in the feed mixture. However, in order for selective separation of the one or more desired gases by the use of separation membranes to be commercially attractive, the membranes must satisfy several criteria so that the use of the separation procedure is economically attractive. For instance, the membranes must be capable of withstanding the conditions to which they may be subjected during the separation operation. The membranes also must provide an adequately selective separation for one or more desired gases at a sufficiently high flux, that is, permeation rate of the permeate gas per unit surface area. Thus, separation membranes which exhibit adequately high selective separation but undesirably low fluxes, may require such large separating membrane surface area that the use of these membranes is not economically feasible. Similarly, separation membranes which exhibit a high flux but low selective separation are also commercially unattractive. Accordingly, work has continued to develop gas separation membranes which can provide both an adequately selective separation of one or more desired gases such as acid gases from non-acid gases at a sufficiently high flux for an extended period of time under adverse environmental conditions such that the use of these gas separation membranes is economically feasible.
In general, the passage of a gas through a membrane may proceed through pores, i.e. continuous channels for fluid flow in communicational at both feed and exit surfaces of the membrane which pores may or may not be suitable for separation by Knudsen flow and diffusion; in another mechanism, in accordance with current views of membrane theory the passage of a gas through the membrane may be by interaction of the gas with the material of the membrane. In this latter postulated mechanism, the permeability of a gas through a membrane is believed to involve solubility of the gas in the membrane material and the diffusion of the gas through the membrane. The permeability constant for a single gas is presently viewed as being the product of the solubility and diffusivity of the gas in the membrane. A given membrane material has a particular permeability constant for passage of a given gas by the interaction of the gas with the material of the membrane. The rate of permeation of the gas, i.e. flux, through the membrane is related to the permeability constant, but is also influenced by variables such as membrane thickness, partial pressure differential of the permeate gas across the membrane, the temperature and the like.
Polymers useful as practical membranes for gas separation applications must satisfy a number of stringent criteria. Foremost among requirements are the polymers intrinsic transport properties such as permeability and selectivity. Additional requirements include adequate thermal and chemical-environmental stability and appropriate properties such as solubility characteristics which are crucial to the fabrication of the polymers into useful membranes. At present, most polymers which have been utilized for gas separations belong to the general family known as glassy polymers. For the most part, these materials are attractive because they satisfy very well the above criteria for fabrication into useful membranes in the asymmetric morphology either as film or hollow fiber. However, many polymers which satisfy fabrication criteria possess transport properties which are less than ideal for a given separation application. Frequently polymers which exhibit a desirably high selectivity for a particular gas pair do not allow the faster gas to permeate at an adequate rate. Conversely, polymers with very high permeabilities for a given gas, often are only moderately selective. It is a difficult task to find a single material which will simultaneously satisfy most or all of the necessary requirements for the desired gas separations.
Glassy polymers are generally highly amorphous materials, which are, as their name implies, in a frozen state at ambient temperatures. Above the glassy transition temperature or T.sub.g of the polymer, the glassy solid changes into another amorphous solid state, a rubber which then is characterized by much more rapid motion on the molecular scale of the polymer chains. Of particular interest among the various properties which distinguish polymers in the rubbery state versus the glassy state is that the transport properties are often drastically different for the two types of materials. Permeabilities for gases through many rubbers are very high compared to permeabilities of the same gases in many glassy polymers. However, the more dynamic nature of the polymer chains in the rubbery state, which is generally responsible for the higher permeabilities, often causes much lower selectivities for rubbery polymers as compared to many glassy polymers. In addition, many rubbery polymers do not possess an appropriate combination of other properties required for efficient fabrication into membranes having the preferred asymmetric morphology.
Some rubbery polymers have been and are being used in gas separations. Silicone rubbers have been applied to air (O.sub.2 /N.sub.2) separations, particularly for small scale uses such as blood oxygenation or air oxygen enrichment. In such a circumstance, it is the very high O.sub.2 permeability of silicone rubbers which outweights less attractive properties, such as low selectivity and mechanical weakness. Since the silicone rubbers cannot readily be made in asymmetric form, the polymer is supported typically on a relatively strong porous support. Such porous supports can in appropriate applications effectively circumvent a rubbery polymer's limitations regarding fabrication and mechanical strength. For large scale gas separation applications of potential commercial importance, it remains, however, that the usually inadequate selectivity characteristics of most rubbery polymers limit their practical utility.
Polyphosphazenes are polymers having a phosphorous-nitrogen sequence with organic substituents on the phosphorous as follows: ##STR1## where R and R' are the same or different organic substituents and n is an integer of ten or more.
A limited number of single gas transport measurements of polyphosphazenes has been made. For instance, Bittirova, et al, Vysokomol. Soedin, Ser. B, 23(1), 30-3 (1980) discloses the permeability to oxygen, nitrogen and argon of poly(octyloxy phosphazene). The Bittirova, et al reference focuses on one polyphosphazene with particular interest in the material because of its "specific properties" including the translucent, flexible, elastic films having permeability coefficient values for O.sub.2, Ar, N.sub.2 of 12.84.times.10.sup.-7, 11.88.times.10.sup.-7 and 5.25.times.10.sup.-7, cm.sup.3.cm/cm.sup.2.s.atm, respectively. The reference makes no attempt to qualify the elastic films further with regard to other gas transport properties.
Kireyev et al, Vysokomol. Soedin, Ser. A18(1), 228 (1976) and Chattopadhyay, et al, J. Coating Technology, 51 (658), 87 (1979) disclose water vapor permeability in poly(butyloxy phosphazene) and in poly(aryloxy phosphazenes) respectively. Kireyev, et al discusses the need for new types of elastomers; thus, the interest in polydiogano phosphazenes (one of the qualifying physical property studies relates to the absorption of steam by these phosphazenes as examined gravimetrically). Chattopadhyay, et al provides a publication entitled "Polyphosphazenes As New Coating Binders" with special interests in the polyaryloxy phosphazenes as a material having a high degree of flame retardancy and other desirable polymeric properties for application as paint binders. The Chattopadhyay, et al reference along with other physical test evaluations indicate moisture vapor transmission through the polymeric film at 25.degree. C. No mention of the separation of a gas mixture by polyphosphazene membranes has been made.
In summary, suitable polyphosphazene gas separation membranes have not been provided. Particularly, the suitability of polyphosphazene gas separation membranes for separation of acid gases from non-acid gases has not been suggested.